The goal of vegetable breeding is to combine various desirable traits in a single variety/hybrid. Such desirable traits may include greater yield, resistance to insects or pests, tolerance to heat and drought, better agronomic quality, higher nutritional value, growth rate and fruit or pod properties.
Breeding techniques take advantage of a plant's method of pollination. There are two general methods of pollination: a plant self-pollinates if pollen from one flower is transferred to the same or another flower of the same plant or plant variety. A plant cross-pollinates if pollen comes to it from a flower of a different plant variety.
Plants that have been self-pollinated and selected for type over many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny, a homozygous plant. A cross between two such homozygous plants of different varieties produces a uniform population of hybrid plants that are heterozygous for many gene loci. Conversely, a cross of two plants each heterozygous at a number of loci produces a population of hybrid plants that differ genetically and are not uniform. The resulting non-uniformity makes performance unpredictable.
The development of uniform varieties requires the development of homozygous inbred plants, the crossing of these inbred plants, and the evaluation of the crosses. Pedigree breeding and recurrent selection are examples of breeding methods that have been used to develop inbred plants from breeding populations. Those breeding methods combine the genetic backgrounds from two or more plants or various other broad-based sources into breeding pools from which new lines are developed by selfing and selection of desired phenotypes. The new lines are evaluated to determine which of those have commercial potential.
One crop species which has been subject to such breeding programs and is of particular value is the pea. Peas are the round embryos which grow in a pod of a leguminous vine, belonging to the family Leguminosae (Fabaceae). The most well known species is Pisum sativum L., which includes the edible garden pea, a vegetable, and the field pea—grown especially for animal feed. Depending on taxonomic interpretation, other members of the genus Pisum L. include Pisum elatius Steven ex M. Bieb. (wild pea), Pisum fulvum Sibthorp & Sm. (tawny pea), and Pisum syriacum Lehman. (Syrian pea). There are many varieties of garden peas. These can be grouped into those with low-fiber, edible flat pods, such as snow peas and sugar peas, those with low-fiber, edible, thick pods, such as snap peas, and those with fibrous stringy pods, intended to be shelled for the edible embryos inside, such as English peas.
Pea plants are able to reproduce by self-fertilization and cross-fertilization. Thus far, however, commercial pea varieties have been inbred lines prepared through self fertilization (McPhee, 2005).
Peas are one of the top vegetables used for processing in the United States; with approximately 90% of the grown pea acreage used for processed consumption (NASS Census of Agriculture 2002). The pea is an annual cool season plant, growing best in slightly acidic soil. Many cultivars reach maturity about 60 days after planting. Pea plants can have both low-growing and vining cultivars. The vining cultivars grow thin tendrils from the leaves of the plant, which coil around available supports. The pea pods form at the leaf axils of the plant.
As with other legumes, pea plants are able to obtain fixed nitrogen compounds from symbiotic soil bacteria. Pea plants therefore have a substantially reduced fertilizer requirement compared to non-leguminous crops. This advantage adds to their commercial value, particularly in view of increasing fertilizer costs, and has generated considerable interest in the creation of new pea plant cultivars.
While breeding efforts to date have provided a number of useful pea lines with beneficial traits, there remains a great need in the art for new lines with further improved traits. Such plants would benefit farmers and consumers alike by improving crop yields and/or quality.